The state of the art relating to the manufacture of such dies consists in particular in depositing on a glass support or substrate, a photosensitive resin layer, and in then recording information on this resin layer by means of a beam of an appropriate radiation, in particular a laser beam, in developing this layer, that is to say removing the parts of the latter where the information has thus been transferred so as to uncover the support in the corresponding zones, and in then creating in this support through the parts thus uncovered, micro-pits which correspond to the signal to be reproduced.
Having extracted the remaining resin, the unit constitutes a master disk from which may be obtained dies or replicas reproducing these micro-pits in relief, these dies later allowing, by pressing a material in particular of the polycarbonate kind, the production of disks which in their turn carry as depressions the pits representing the information to be read on these disks. In particular the master disk may usefully be coated with a thin metal layer, generally silver, which follows the profile of the micro-pits, before being itself covered by electroplating with a complementary nickel layer which will constitute the desired die, whilst offering projecting micro-reliefs corresponding to the micro-pits previously formed in the substrate and which, replicated in the final disk, will be readable by the customary optical methods.
A major disadvantage of this manufacturing technique resides in the need to irradiate the photosensitive resin with a laser beam, thus entailing the use of a very sophisticated machine being useable exclusively by professionals and, moreover, requiring very heavy maintenance. In particular, the laser beam must be guided by a very complicated mechanical and optical unit so as to provide, with extreme accuracy and perfect reproducibility, the exactly centred continuous spiral which will create, in the support, the very narrow imaginary groove along which the successive micro-pits are arranged.
Similarly, there is known the process for recording optical disks of the "WORM" (Write once, read many) type which consists in recording information directly on a polycarbonate substrate covered with a sensitive layer with the aid of a straightforward laser diode, it thus being possible to carry out the writing performed on the disk by means of a simple apparatus, widely available to the public and of low cost.
The "WORM" type disks are thus made up of a pre-etched substrate covered with one or more sensitive layers which are modified in contact with the beam issuing from the laser diode, in particular while producing by thermal ablation the micro-pits representing the information to be recorded.
Such disks in which the modification of the sensitive layer is accompanied by a variation in the reflectivity of the material constituting it are in particular described for example in the American patent U.S. Pat. No. 4 414 273, European patent EP-A-0 130 026 or again in the journal "Optical memories and systems" No. 72 - March 89, page 5 et seq. The recording of these disks is straightforward, easy to use even by a non-professional and requires only a relatively uncomplicated apparatus.